How much of a danger is this oil leak?August 23, 2013 9:01 AMSubscribe

I found out last night that my car is leaking oil onto my exhaust system. (That certainly explains the hot oil smell I've been smelling.) The guy at the lube shop said I should only use the car for short-distance driving until I can get it fixed, lest the exhaust get hot enough to cause the oil to catch on fire. Complication: I was planning taking a 45-mile trip this evening (and a 45-mile trip back afterward). I would hate to have to cancel. How big of a risk is this, really?

It's a 2003 Nissan Sentra. The trip will take anywhere from 60 to 90 minutes, depending on traffic and stops.

So, I guess I'm wondering:

1. How likely is this to happen? About half of the trip is highway driving, which should help to air-cool the exhaust system, right?

1. I doubt there is enough airflow over the exhaust to cool it sufficiently below the combustion point of oil, even at highway speeds. Cars are designed to cool the radiator with air, not the entire engine compartment. I wouldn't risk it.

2. You're looking at the car burning up entirely, hopefully with you outside it.posted by exogenous at 9:10 AM on August 23, 2013

How much oil and from where ?

I don't want to say there is no chance it will catch fire, but, look, I've driven thousands of miles 60s and 70s model cars that only stopped leaking oil when they were out. So, not to be too cavalier with someone else's life and property - but I probably wouldn't worry too much about it if I were in your shoes.

The bigger worry is what this leak means - if you're leaking oil from your head gasket, then... you've got much bigger worries. If it's the valve cover gasket - is it leaking or seeping ? And, are you sure it's not from a spill or loose cap ?

Anyway, on the face of it, I wouldn't be too worried about it. But you have better information than me.posted by Pogo_Fuzzybutt at 9:18 AM on August 23, 2013 [1 favorite]

IT all depends. I have dealt with spilled oil on the exhuast manifold many times and it has never caught fire...however it could have. Motor oil can burn, but it is hard. I think the flash point is like 500 deg F or more.

On several Honda engines made through the 90's the oil filter is directly above the exhaust manifold and it is almost impossible to change it without spilling at least a few drops on the exhaust pipes. Honda's with this engine (or any other engine) don't have a reputation for catching on fire. So if it is just a minor leak of a few drops it isn't much to worry about, if it is a significant leak with a steady flow of oil it might be a problem and not just from fire. A big enough leak can drain the oil from the car and not leave enough to keep the engine well lubricated and alive so definitely keep an eye on the oil level.posted by bartonlong at 9:40 AM on August 23, 2013

The oil level is fine (and I haven't topped it off since my last oil change back in April), so the leak must not be that bad. (I don't do much driving; I've only put a thousand-odd miles on the car since the last oil change, and the guys at the lube shop last night said that everything is fine and I don't need to change it until 3,000 miles have elapsed.)posted by escape from the potato planet at 9:49 AM on August 23, 2013

Given that you haven't even leaked a measurable amount in 1000 miles I'd guess you are safe to drive. However, I would stop by the auto parts store and buy a fire extinguisher to keep in the car. Just in case.

At this distance and with this level of information, we can't really judge how safe or dangerous this is, or how likely/unlikely it is that your Sentra will catch fire. We can guess, but that's about all we can do. Knowing where the leak is coming from, the rate of leakage, and exactly where on the exhaust system the leak is hitting would give us the ability to more precisely gauge the actual risk, but even then, we're still sort of a guessing.

Based on what you've told us, this is probably safe, if a bit smelly. I've had cars that (at times) leaked as much as a quart of oil every 1k miles. It wasn't leaked directly onto the exhaust manifold, but with that much loss oil got everywhere, and those cars still didn't burst into flames. YMMV, as they say.

On the other hand...it's one thing to smell it, it's another if you have James Bond levels of smoke pouring out from under your hood. You're probably OK if it's the former, but if it's the latter, I would get it fixed before I drove it.posted by mosk at 10:07 AM on August 23, 2013

I haven't noticed any smoke at all (and I've been smelling it for about three weeks).

I guess I'm leaning toward "everything's fine" at this point. The guy at the lube shop seemed pretty serious, but he was also a kid, so maybe he doesn't know what he's talking about.

I wouldn't drive it that far. Leaks never get better over time, they get worse. Is the source of the leak known or is there a way to find that out? Even a stop-gap leak repair should be enough for a short time until you can get it properly fixed.posted by tommasz at 10:44 AM on August 23, 2013

This is a small amount of oil you are talking about, not a pool. The flash point of oil is fairly high, meaning that the oil is really unlikely to burst into flames. The draft of your moving car will also disburse the heat and fumes, so there is, again, not a large risk of fire.

The main reason to get this fixed is that the fumes are not healthy for you, and oil particles are accumulating inside your car. This will make it less pleasant to be in, over time.posted by Midnight Skulker at 10:53 AM on August 23, 2013

//Leaks never get better over time, they get worse//

My 97 Mazda 626 has leaked oil for about 5 years - at a consistent rate of a quart every 1000 miles if I don't use Stop Leak to swell the seals and minimize the leak. The car has 190K miles on it, I'm not about to spend big bucks on it and my mechanic agrees that band aiding it is fine as long as the leak doesn't get more severe. I use Stop Leak at each oil change and park over a pile of kitty litter to keep the leak from damaging my driveway. 10 year old cars can have minor oil leaks that never really get worse, depending on the cause. Granted, we don't know the cause here, but if it is just a seal getting old there may never be a mechanical need to fix it. The smell and dripping onto the exhaust may be a reason to fix it though.posted by COD at 10:53 AM on August 23, 2013

Is it possible for you to get a rental car or something? Given how this happened lately, I think I wouldn't enjoy the trip much if I kept wondering if my car was gonna be set on FIRE.posted by jenfullmoon at 11:04 AM on August 23, 2013 [1 favorite]

escape from the potato planet: "The oil level is fine (and I haven't topped it off since my last oil change back in April), so the leak must not be that bad. "

Something here sounds fishy. Did they make an offer to fix this "problem"? Because lube shops have a bad habit of spilling oil onto the exhaust while changing it, which would cause some temporary smoking but is mostly harmless. If it's not losing oil then something is up.

Although you can have very small seeping leaks around valve cover gaskets that will seep oil onto the exhaust manifold. I've never heard of one bad enough or the exhaust getting hot enough to produce flame.posted by Big_B at 12:09 PM on August 23, 2013

Woah, my partner has the exact same car. And it had the same issue. It was the valve cover gasket, and the shop mentioned they had seen that issue on other sentras of a similar year range/that generation as i remember. Just for some random data for you to stow away, replacing the gasket and a belt tensioner that had toasted itself and a couple serp belts only totaled up to about $500.(and i do my own repairs on my car, and looked at it. it was definitely a Pain In The Ass on this particular car and was definitely more labor than parts)

So here's the rub. The engine seems to tilt a bit forward in these, almost like a lot of motorcycles. Also in that vein, the exhaust manifold is on the front of the engine. See where i'm going with this? even a tiny leak will drip straight on to what is essentially the hottest part of the car. There's a heat shield over the header/manifold, but it starts a few inches away from the engine... so the oil drips right past it on to the bare, incredibly hot pipes. To address your #1 point, the manifold+heat shield is right behind the grille and radiator, but it's not like i drove the thing around with the hood open to see how much air makes it through there. That seems to be a secondary intention of the design, and is also not really applicable to this situation since it's such a small amount of oil.

The thing is, if you aren't actually noticing oil loss it's probably a TINY amount of oil. It was on my partners car, just enough to smoke and smell like shit but never enough to actually change the oil level even over the 3000 miles between oil changes.

I drove the car this way for a lot of 120+ mile round trip nearby city and back trips to visit friends, often going 70+mph on the highway at close to 3000rpm. The smell would show up for a bit, burn off, and come back when i stopped for a bit at a restaurant or something cyclically. It never caught fire, because there wasn't enough oil for there to do so. The oil level not dropping and there being enough oil to cause a serious problem are pretty much mutually exclusive here.

On preview, i disagree with Big_B that they're probably making this up. I do however think it's the tiny seeping leak they're talking about, because that's exactly what was happening on my end. Enough to stink and smoke a tiny bit, but not enough to do much else.

I, personally would take this trip myself in your car and be like 8/10 confident that nothing at all was going to happen, and 9/10 confident that if anything did happen it wouldn't really matter. This is based almost entirely on the fact that your oil level isn't actually dropping a measurable/meaningful amount by the way.

I will note i guess, out of some sense of obligation that i'm from the same school of thought as pogo_fuzzybutt. I drive a mid 60s car myself that leaks oil and even weeps gas from the carb. I just threw a halon fire extinguisher under the seat and went "meh". And that meh has lasted well... a couple years at this point. As far as i can tell i haven't exploded in to a fireball yet, and if i have then this afterlife is a lot more boring and tedious than i thought it would be.posted by emptythought at 12:20 PM on August 23, 2013 [1 favorite]

I guess I'm leaning toward "everything's fine" at this point. The guy at the lube shop seemed pretty serious, but he was also a kid, so maybe he doesn't know what he's talking about.

They have to err on the side of caution. 99.99% of minor leaks like this never cause a problem but if he said "you'll be fine, no need to get it fixed" and your car did catch fire they'd be sued big time.

Assuming you don't have any other combustible material at the leak site (like leaves or a mouse nest) I wouldn't sweat a weepy leak onto an exhaust manifold at all. If it was a significant leak onto the catalytic convertor (cats are way hotter than the exhaust manifolds) where a significant amount of oil could accumulate while the car was off I might be worried but there is essentially no risk from a tiny bit of oil on the exhaust manifolds.

Carrying a fire extinguisher is always a good idea. Be aware that you should never open the hood on an engine fire.posted by Mitheral at 1:44 PM on August 23, 2013

If the oil doesn't need significant and frequent topping off, I really think this isn't worth worrying about and definitely worth getting a second opinion on.

Any time any of my trusted mechanics have noted any oil leaks, specifically onto the exhaust manifold or pipes, they have said it's really nothing to worry about. When I asked my MBZ mechanic about possibly getting a new valve cover gasket because so much oil appeared to be seeping out and onto the exhaust manifold he said it was such a slow leak and the amount of oil was very small and it wasn't worth fixing it.

Yeah, I haven't been a regular driver for 12+ years. I'm mechanically savvy, but very little experience with cars. I've been driving an 18 yo "sports" car for commute for the last 7 months.

Doesn't sound like an acute problem, you should be fine barring accidents or force majeure. A 45 mile trip in a 2003 car isn't going to be a problem.

Still, if you intend to keep your car for another couple of years, get the source - not the symptoms - fixed. Don't go to the lube guys, maybe search for mechanics in your area (google maps; plug in your location ,search nearby, read reviews) and see if someone is willing to tell you in jargonlesslower speak.

A proper mechanic will be able to figure out where the leak (if any) is coming from, and fix it. No leak, no more oil drip, no more burning oil smell.posted by porpoise at 9:33 PM on August 23, 2013

(Belated) thanks to all. For the record, I ended up making the drive, and everything was fine. I still haven't gotten around to getting it fixed (I swear I'm going to make an appointment tomorrow!), and everything is still fine.posted by escape from the potato planet at 5:28 PM on December 2, 2013

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